Since the 1960's it has been common for residential refrigerators to have a built-in defrost heater system. The defrost heater system prevents ice crystals from building up on the evaporator coils, which hampers the heat transfer capabilities of the coil, thereby overworking the compressor and eventually shortening the life of the unit.
The typical defrost heater system pioneered by General Electric includes one or more quartz tubes, each with a resistive heating element suspended therein and an end cap at either end. The end cap not only provides weld tab to keep the heating element suspended, but it also provides a means for attaching power leads to the heating element. Additionally, the end cap must keep as much moisture out of the tube as possible, as moisture can corrode and thus shorten the life of the heating element, or it can also corrode the contact in the end cap.
The end cap must also provide a vent for the defrost heater system. When the heating element is energized, the rise in temperature causes a sudden increase of the volume of the gas inside the quartz tube. The vent allows the gas to be released to the outside atmosphere. When the heating element is deenergized the opposite phenomenon occurs. As the temperature in the tube decreases, a partial vacuum forms in the tube. The vent allows outside air to enter the tube to relieve the vacuum. The rate at which air can flow in or out of the quartz tube is a function of smallest cross-sectional area of the vent. Prior art end caps typically have vents that terminate as a single hole in the throat of the end cap, where the quartz tube is received. Sometimes this is insufficient to provide the necessary venting depending upon environmental variables. If there is insufficient airflow the pressure differential between the air inside the quartz tube and the ambient air can cause the tube to break, and hence the heater system to fail.
As mentioned above, the end cap must keep moisture out of the quartz tube. For this reason, it has been known to place moisture seals or barriers around the point where the power leads enter the end cap. Such an end cap is shown in U.S. Pat. No. 3,280,581 to Turner, which is somewhat similar to the end caps that are still used today. Occasionally, these seals degrade and moisture which has condensed on the power supply wires will drip down the wire and into the end cap, thus providing additional modes of failure for the heater system.
Thus it is an object of the present invention to provide an end cap for a defrost heater system which provides increased airflow between the quartz tube and the ambient air.
It is a further object of the present invention to provide such an end cap with improved means of preventing moisture from entering the quartz tube.
It is also an object of the present invention to provide such an end cap at a reduced cost of manufacture compared to presently used end caps.